Selective catalytic reduction (SCR) systems may be used in a vehicle to facilitate reduction of engine output NOx by a reductant, such as urea or ammonia. An SCR system involves injecting the reductant upstream of an SCR catalyst where the reductant, or reductant products, can react with NOx to create byproducts such as nitrogen and water. However, there is a trade-off between an amount of reductant injected and NOx conversion efficiency with SCR systems. Namely, if a large amount of reductant is injected, a great amount of reductant may be stored on an SCR catalyst, and thus there may be high NOx conversion efficiency. However, there is also a risk of reductant “slipping” to an exhaust tailpipe. On the other hand, when a lesser amount of reductant is injected, NOx conversion efficiency decreases and NOx emissions may be emitted through the exhaust tailpipe.
A NOx reduction system having a first and second catalyst bed in series is described in U.S. Patent Application US 2005/0284134A1 (Radhamohan et al). The system includes a first ammonia injector positioned upstream of the first catalyst bed and a second ammonia injector downstream of the first catalyst bed and upstream of the second catalyst bed. The first ammonia injector is configured to inject an amount of ammonia that is less than a stoichiometric ratio, so as to reduce only a portion of the NOx received at the first catalyst bed. The second injector is controlled to inject an amount of reductant so as to remove a portion, or all, of the remaining NOx that flows to the second catalyst bed from the first catalyst bed.
In contrast, Applicants have developed systems and methods for manipulating performance of several regions of an SCR system by controlling reductant delivered upstream of a first SCR region. One exemplary emission control system of a vehicle includes a first SCR region upstream of a second SCR region. An exemplary method for controlling the emission control system includes, in a first mode, adjusting an amount of reductant delivered upstream of the first SCR region based on a condition of the first SCR region. The method further includes, in a second mode, adjusting the amount of reductant delivered upstream of the first SCR region based on a condition of the second SCR region. The first SCR region may be positioned upstream with respect to the second SCR region, in one example.
By selectively adjusting upstream reductant delivery in this way, it is possible to adjust the operation and performance of each of a first and second SCR regions to provide different modes of operation, depending on operating conditions. In this way, a high NOx conversion efficiency can be maintained while controlling a risk of reductant slip over a wider range of operation conditions, without requiring independent control of reductant delivery to each SCR region. For example, if the upstream region has reduced capacity, it may be desired to provide a higher amount of storage in the downstream region and utilize NOx conversion in the downstream region to a greater extent. Here, reductant delivery may be adjusted based on the storage level of the downstream SCR region. However, if the upstream region has increased capacity, it may be desired to provide a higher amount of storage in the upstream region and utilize NOx conversion in the upstream region to a greater extent. Here, reductant delivery may be adjusted based on the storage level of the upstream SCR region. In this way, overall performance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.